Add CLI/MCP parity spec

[obj-p]

Summary

• Proposes a daemon-based architecture to close the CLI/MCP capability gap — every MCP tool gets a CLI equivalent
• CLI becomes a thin JSON-RPC client over a Unix domain socket at ~/.previewsmcp/serve.sock
• Single previewsmcp serve --daemon process multiplexes N concurrent sessions (ADB-style)
• Implementation plan: 10 stacked PRs, each independently mergeable and validated

Key decisions captured

• Transport: Unix domain socket (no port conflicts; filesystem-scoped permissions)
• Session discovery: RPC-only, no per-session files. Daemon is single source of truth.
• Lifecycle: Docker-style — attached run by default, --detach opt-in, stop for cleanup
• Resolution: --session / --file / sole-session default with clear ambiguity errors
• Reuse MCP SDK: configureMCPServer() is already transport-agnostic; swap StdioTransport for Unix socket transport

Related

Closes neither — those stay open until implementation lands:

• CLI/MCP parity: run command lacks interactive capabilities #94 — CLI/MCP parity (broader goal)
• CLI run command has no way to capture a snapshot of the running preview #92 — snapshot from running session (subset)
• Interactive REPL mode for run sessions #95 — interactive REPL (deferred follow-up)

Test plan

• Review spec sections for completeness
• Validate architectural assumptions (especially `NWEndpoint.unix(path:)` + `NetworkTransport` fit) during PR 1

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